Fierro literally translates to "iron", but in this song, it's used as popular Mexican slang. It's a very enthusiastic way of saying "yes", "let's do it", or even "hell yeah!".
In the lyrics, Cazzu creates a cool call-and-response: "Yo te digo 'vamo', tú me dice' 'fierro'" (I say 'let's go', you say 'hell yeah!'). It's a short, punchy word that captures a rebellious and confident energy, perfect for learning real-world, colloquial Spanish.
Cazzu kicks this song off with one of the coldest lines ever: "Perdón si no, no te llamé / Es que me acuerdo de tu carita, pero tu nombre me lo olvidé" (Sorry if I didn't call you / I remember your face, but I forgot your name). She's laying out her rules from the very beginning. She's not looking for love and makes it clear she's not the romantic type, warning a potential lover that her "corazón [está] echado a perder" (heart is ruined) and that she doesn't believe in true love. She wants fun on her own terms and nothing more.
She's all about her independence and career, working 24/7 to make her own money. She rejects the idea of being 'won over' with a powerful metaphor: "Yo no soy país pa' que vengan y me conquisten" (I'm not a country for you to come and conquer me). She wants someone to have a good time with, but the most important thing is that when she wakes up, "estar segura que de mi casa te fuiste" (to be sure you've left my house). It's a bold statement about wanting freedom over commitment and living life exactly how she wants.